Vehicle systems may include an engine with an exhaust gas treatment system coupled in an exhaust passage in order to control regulated emissions. In some examples, the exhaust gas treatment system may include a selective catalytic reduction (SCR) system in which a reductant, such as urea or ammonia, is added to the exhaust stream upstream of a reduction catalyst such that NOx may be reduced by the catalyst. In such an example, the reductant (e.g., urea) may be delivered to the exhaust passage via a reductant injector fluidically coupled to the exhaust passage. Reductant injectors are, however, prone to leakage. In addition, due to poor reductant dosing control, reductant (e.g., ammonia) may be systematically over-injected, leading to formation of deposits in the exhaust passage and the injector nozzle.
One approach for reducing deposition of ammonia in urea or ammonia injection systems is shown by Yacoub et al. in US 20120090296. Therein, a concentration of NOx and ammonia in the vehicle exhaust system, downstream of the reductant injector, is estimated or modelled. In response to the detected ammonia level being higher than a desired ammonia level, ammonia deposition is determined and addressed by heating the exhaust system to purge the deposited ammonia. For example, exhaust gases are heated via spark timing retard and increased engine throttling.
However, the inventors herein have identified potential issues with such an approach. While the engine heating approach of Yacoub et al. addresses ammonia deposition during engine operation, ammonia deposits may linger and continue to release ammonia long after the vehicle has been turned off. Specifically, based on the severity of injector leakage, as well as the degree of dosing error, ammonia deposits may continue to form even after an engine has been shutdown to rest and the vehicle has been turned off. The continued presence of ammonia deposits releases ammonia vapors from the deposits via natural sublimation. These vapors can plug the reduction catalyst, lowering the catalyst's efficiency during subsequent engine operation. In addition, exhaust emissions may be degraded.
In one example, some of the above issues may be addressed by a method of identifying reductant injector leakage. The method comprises, indicating degradation of an exhaust reductant injector based on an output of an exhaust NOx sensor following engine shutdown to rest. In this way, reductant injector health can be correlated with the lingering presence of ammonia deposits after a vehicle engine has been turned off
For example, an engine system may be configured with an SCR catalyst in the exhaust passage and a urea injector positioned upstream of the SCR catalyst. A feedgas NOx sensor may be coupled to the exhaust passage upstream of the SCR catalyst and upstream of the urea injector. Optionally, an additional tailpipe NOx sensor may be coupled to the exhaust passage, downstream of the SCR catalyst. During an engine shutdown to rest, a controller may estimate an amount of unreacted ammonia that remains in the exhaust system, for example in an exhaust passage puddle, and/or stored in the SCR catalyst. Further based on ambient temperature conditions and exhaust temperature conditions, the controller may determine an ammonia profile expected in the exhaust passage (between the reductant injector and the SCR catalyst) including ammonia levels expected over a duration since the engine shutdown. The controller may then use the feedgas exhaust NOx sensor as an ammonia sensor during the engine shutdown to monitor exhaust ammonia levels in that region of the exhaust passage. If the detected ammonia profile matches the expected profile, no reductant injector issues may be flagged. However, if the profiles do not match, for example, if the estimated ammonia level is higher than the expected level, and/or if the elevated ammonia levels persist longer than expected, the presence of excess ammonia (e.g., larger ammonia deposits) may be confirmed, and injector degradation may be indicated.
In this way, the health of a reductant injection system can be better diagnosed. By using a feedgas exhaust NOx sensor, injector leakage and reductant deposit issues that persist during engine shutdown can be detected and accordingly addressed. Specifically, the lingering presence of elevated ammonia levels in an exhaust passage (upstream of an exhaust catalyst and downstream of a reductant injector) after an engine shutdown can be correlated with the presence of urea injector leaks, and ammonia deposits in the exhaust passage. By using the feedgas exhaust NOx sensor during engine-off conditions to detect reductant sublimated from exhaust passage reductant deposits, component reduction benefits are also achieved. Overall, exhaust emissions are improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.